Doors, hoods, fenders, tail gates, trunk and deck lids of motor vehicles are usually formed by an outer sheet metal panel which is joined to an inner reinforcing panel around its periphery by hemming a flange around the periphery of the outer panel over an adjacent edge of the inner panel to secure the panels together.
Typically, this hemming is accomplished in two stages. In the first stage, with the reinforcing panel nested in the outer panel and received in a suitable fixture, a prehemming steel or tool engages and bends an upturned peripheral flange of the outer panel to an acute included angle with respect to an adjacent portion of the outer panel. Thereafter, a second hemming steel engages and completely bends the prehemmed flange of the outer panel over the peripheral edge and into engagement with the reinforcing panel to securely attach the panels together in a unitary structure. Typically, a plurality of both prehemming and final hemming steels or tools actuated by presses are grouped around the periphery of the outer panel to perform all prehemming and hemming operations for one assembly of an outer and a reinforcing panel.
For a typical assembly, the flange around the outer panel can be formed at substantially a right angle to the adjacent portion of the outer panel which is necessary for a satisfactory prehemming operation. However, if the outer panel has a cambered or inclined portion adjacent to its periphery, when the flange is formed, such as by sheet metal stamping, it may be at an obtuse included angle to the immediately adjacent cambered portion of the panel. This situation frequently occurs in the portion of the deck lid adjacent the rear window and quarter panel area of a vehicle and in the area of the hood adjacent the front window and fender portion of the vehicle. Previously, any portion of a flange at an obtuse included angle had to be bent to an acute included angle by striking it either manually with a hammer or with an automated wiper tool actuated by a cylinder both of which produce inconsistent results and result in a completed hem of poor quality appearance of the bend and an unacceptable fold line around the periphery of the outer panel. Typically, this fold line is so wavey and of such poor quality that surface filling and fairing operations are required to provide a suitable exterior surface for painting or other finishing of the panel. This is costly, inefficient and ineffective.